Each question below has two blanks, which is indicating

Solution

The sentence discusses the difficulty in evaluating a systematic review based on the types of studies it includes.

• The first blank refers to a type of study that presents a "second difficulty." Observational studies are a category of research that often have varying methodologies and can be more prone to bias than randomized controlled trials. This inherent variability makes them harder to assess in a systematic review.
• Uniform means remaining the same in all cases and at all times; consistent. This word accurately describes the standardized and highly controlled methodology of RCTs, which makes them easier to assess in a systematic review.